XINING, June 13 (Xinhua) -- More than 500 square kilometers of selenium-rich soil, which can be used for growing healthy selenium-rich plants, has been found in northwest China's Qinghai Province, local authorities said Tuesday.

The selenium level in the soil ranges from 0.23 milligram per kilogram to 2.8 milligram per kilogram, with an average level of 0.4 milligram per kilogram, according to the Land and Resources Department of Qinghai Province.

Experts said the selenium-rich soil is clean without heavy metal pollution, which means it can be used to develop environmentally friendly selenium-enriched farm produce.

Selenium is a chemical element and an important micronutrient. It can boost the immune system, slow down the aging process, and reduce the effects of cardiovascular diseases, according to Tian Xike from China University of Geosciences.

Selenium reserves are relatively low in China. Selenium-rich soil has only been discovered in around four provinces, Tian said.